1. Field of the Invention
The present invention relates to a mobile communication system formed by base stations and mobile stations, and more particularly, to a channel allocation scheme used by each base station in establishing communications with mobile stations.
2. Description of the Background Art
A well known type of the mobile communication system currently in practical use as a portable telephone system or an automobile telephone system is a cellular system in which a communication service is provided by distributing a number of base stations over its service area, where each base station serves for a radio zone with a radius of about several km.
In general, in order to utilize a finite amount of available radio channel resources efficiently, this type of a mobile communication system adopts the geographically repeated utilization of the radio channels in which the identical radio channel is used by more than one geographically distanced base stations. In this case, by making a channel reuse distance between the base stations which uses the identical radio channel as small as possible, the higher frequency utilization efficiency can be achieved, so that the system capacity can be increased within a condition of a constant frequency bandwidth given to the system.
However, this channel reuse distance cannot be made unlimitedly smaller, because when this channel reuse distance is made smaller, there arises a problem of the interference from the identical radio channel or the interference from the neighboring radio channel in a case of an interleave scheme in which each radio channels are formed by allowing overlaps in the power at the side band of each radio channel in an FDMA (Frequency Division Multiple Access) system, which significantly lowers a communication quality. For this reason, there is a need to restrict the repeated utilization of the identical radio channel only among those base stations which are sufficiently distanced spatially to keep the interference within a certain tolerable level.
The schemes for realizing this repeated utilization of the radio channels within such a practical condition include a fixed channel allocation scheme and a dynamic channel allocation scheme, of which the dynamic channel allocation scheme further includes a centralized control type dynamic channel allocation scheme in which a control station for controlling the channel allocations is provided with respect to a plurality of base stations, and an autonomous distributed type dynamic channel allocation scheme in which each base station allocates the radio channels autonomously and distributedly.
The fixed channel allocation scheme fixedly allocates the radio channels to each base station according to the radio wave propagation state and the traffic distribution within the service area obtained either by the actual measurements or by the theoretical calculations. In general, this fixed channel allocation scheme is associated with problems that an enormous amount of efforts are required in obtaining a design for determining the fixed allocations, and that the re-designing is required at a time of the system expansion such as the addition of base stations, so that it has a very low adaptivity to a system expansion. In addition, the available radio channels of the entire system are to be divided into some number of groups and allocated to a number of base stations, so that there is a loss of efficiency of large groups, and therefore it cannot achieve the high frequency utilization efficiency.
On the other hand, in the dynamic channel allocation scheme, it is possible to allocate the radio channels flexibly to some extent in accordance with the temporal variation and the spatial bias of the traffic, and all the available radio channels of the entire system are made to be usable by any base station of the system, so that the large grouping effect can be obtained, and consequently the frequency utilization efficiency can be improved compared with the fixed channel allocation scheme.
However, in order to suppress the call loss rate or the interference probability to the minimum level, an enormous amount of data and quite complicated controls are necessary in general, and in a case of the centralized control type dynamic channel allocation scheme, it requires a considerable amount of signal traffic between each base station and the control station, while in a case of the autonomous distributed type dynamic channel allocation scheme; it requires many processing steps before the actual channel allocation can be made so that the connection delay becomes large. Thus, in the dynamic channel allocation scheme, how to realize a channel allocation scheme capable of achieving a less control load and a higher frequency utilization efficiency at the same time presents an important practical issue to be resolved.
To this end, there has been a proposition of a channel allocation scheme for realizing such an autonomous distributed type dynamic channel allocation by a relatively simple control method, as disclosed in Japanese Patent Application No. 61-244137 (1986) and Japanese Patent Application No. 62-91033 (1967).
In this channel allocation scheme, an allocation priority level of each radio channel is calculated from the past records of the channel use, i.e., records concerning whether each radio channel had allocated or not in the past, and the judgement as to whether each radio channel is to be allocated or not is made sequentially from a radio channel with a highest allocation priority level. Then, when it is judged to be allocated, the allocation of that radio channel is made accordingly, whereas otherwise the judgement for a next radio channel with a next highest allocation priority level is made.
More specifically, the aforementioned Japanese Patent Application No. 61-244137 proposed a radio communication scheme in which each channel is given a priority level which is dynamically determined according to the past records of the channel use and the channels are sequentially used in an order of their priority levels. On the other hand, the aforementioned Japanese Patent Application No. 62-91033 proposed a transmission channel control scheme in which a transmission success rate for each channel is memorized and the channels are sequentially used in an order of their transmission success rates at a time of transmission, while the transmission success rates are updated according to the transmission result.
However, in such a conventional autonomous distributed type dynamic channel allocation scheme, the allocation priority level of each radio channel has been determined by equally weighting all the past records of the channel use, and using a large number of the past records of the channel use, so that there has been a problem that an enormous amount of time is required in order to follow the changes in the radio wave propagation state and the traffic distribution after the system has reached a stationary state.
Here, the changes in the radio wave propagation state and the traffic distribution are caused by newly constructed buildings in the surrounding of each base station, newly constructed base stations, and/or the starting/ending of the operations of movable type base stations, and the conventional autonomous distributed type dynamic channel allocation scheme using the past records of the channel use has been unable to follow such changes of the radio wave propagation state and the traffic distribution, so that there has been problems of large call loss rate, interference probability, and connection delay.
Moreover, in such a conventional autonomous distributed type dynamic channel allocation scheme which calculates the allocation priority level of each radio channel according to the past records of channel use and makes the radio channel allocation according to the calculated allocation priority levels, only one priority level has been assigned to each radio channel, so that the repeated utilization of each radio channel is limited to the repeated utilization in units of the radio zones. As a result, the advantage in the frequency utilization efficiency in comparison with the fixed channel allocation scheme has been limited only to the improvement due to the large grouping effect obtained as all the available radio channels of the entire system are made to be usable by any base station of the system.
There has also been a proposition of a channel allocation scheme for realizing an autonomous distributed type dynamic channel allocation by a relatively small control load, as disclosed in Japanese Patent Application No. 1-306417 (1989).
In this channel allocation scheme, all the radio communication channels are divided into a plurality of channel groups, and in order to use these channel groups in accordance with the reception level in the communication, a lower limit of the reception level required in the communication between the mobile station and the base station using a channel of each channel group is set for each channel group. Then, a radio communication channel to be allocated to a communication set up request occurring in each radio zone is selected to be a channel of the channel group for which the reception level in the communication obtained from the reception level in the radio control channel between the requesting mobile station and the base station satisfies the aforementioned lower limit while the carrier to interference power ratio satisfies the required communication quality condition when this channel is allocated for setting up the requested communication.
However, in this conventional autonomous distributed type dynamic channel allocation scheme, it is necessary to determine thresholds to be used in determining the grouping of different channel groups by means of the thoroughly analysis of the radio wave propagation state and the traffic distribution within each radio zone using the actual measurements or the theoretical calculations, in order to suppress the control load required in the radio channel allocation at the minimum level, to maintain the high frequency utilization efficiency, and to satisfy the required communication quality.
But, such a thorough analysis over all the numerous radio zones existing within the service area of the system requires an enormous amount of time and efforts, and there is also a problem that the re-designing is necessary at a time of the system expansion such as the addition of base stations. Moreover, apart from the system expansion, a large variation in the radio wave propagation state and the traffic distribution can be caused by new construction and destruction of buildings, roads, etc. in the surrounding of each base station, so that the re-designing is necessary in order to cope with such a variation.